Phase modulation schemes are very effective and are therefore widely used in communication systems. A simple example of a phase modulation scheme is quaternary phase shift keying (QPSK). FIG. 1 shows a constellation diagram that illustrates how QPSK maps two-bit digital data to one of four phase offsets. FIG. 2 shows a typical QPSK (or I/Q) modulator used to generate a phase-modulated signal. This modulation technique relies on orthogonal signal vectors to realize the phase offsets—an inherently linear technique, since it depends solely on the matching of these orthogonal signals.
The I/Q modulator provides a straightforward approach to generating phase-modulated signals that's also suitable for more complex schemes such as wideband CDMA and OFDM systems. It's also possible to generate the phase-modulated signals using a phase-locked loop—an approach that offers reduced circuitry and lower power consumption. Unfortunately, the performance and bandwidth of typical phase-locked loop architectures is limited.
It would therefore be advantageous to have a low-power, wide-bandwidth, highly linear phase modulator based on a phase-locked loop architecture that overcomes the limitations of conventional systems.